1. Field of the Invention
This invention relates generally to analog-to-digital converters and, more particularly, to analog-to-digital converters that are to be used in applications requiring a high degree of accuracy.
2. Description of the Related Art
In order to achieve high accuracy in the analog-to-digital conversion of a signal, the use of the standard successive approximation technique has proven to be inadequate. As the additional elements are added for improving the accuracy of the measurement, the errors become cumulative and provide a limit on the accuracy of the components of the system that can not be overcome without excessive effort. Therefore, a somewhat different technique has been used in high precision measurements of the signal. The method consists of providing two slopes, for example, charging and discharging a capacitor, on which the input signal acts. The input signal determines the difference in the two measurements. Therefore, because the relationship of the slopes of the charging current depend on the input signal, the times to reach a predetermined upper and return to a predetermined lower level can be used to provide an accurate measurement of the input signal. This technique is shown in FIG. 1. The T1 time indicates the time, for example, for charging a reference capacitor while the T2 time determines the time for discharging the reference capacitor. A constant frequency signal source can be counted during the charging and discharging periods to determine the effect of the input signal.
While this technique has proven to be effective in determining with precision the value of an input signal, the limitation on the accuracy results from the use of separate apparatus for charging the capacitor and for discharging the capacitor. This difference in apparatus has provided a problem in matching components that has proven to provide an upper limit on the accuracy available by this technique in determining a value for an input signal.
A need has therefore been felt for apparatus that permits positive and a negative slope measurement of high precision using a common capacitor charging and discharging circuit. This apparatus can be used as a precision digital measuring circuit. In addition, because of the problems associated with temperature dependence of components, and particularly the semiconductor components, a need has also been felt to provide temperature stability for the charging and discharging circuit.